1. Field of the invention
The present invention relates to a valve driving mechanism for an internal combustion engine, more specifically to mechanical parts employed for the valve driving mechanism.
2. Description of the Prior Art
In a valve driving mechanism for an internal combustion engine, there has been known a structure having a rocker arm provided at one tip end portion with a roller which is brought into a contact with a cam surface of a cam. With this structure, the roller rotates as it contacts with the cam surface so that a wear amount of the cam and the roller can be reduced in comparison with a structure in which a tip end of rocker arm slidably contacts with a cam surface because a friction coefficient by rolling is smaller than that by sliding. As a result, the structure provided with the roller at the tip end of the rocker arm can improve a durability of the valve driving mechanism.
It should however be noted that, in the valve driving mechanism having the rocker arm provided with the roller, the parts of the mechanism are required to have an improved pitting resistance property as well as an improved wear resistance property.
For this purpose, conventionally, a cam shaft having a cam portion of a chilled structure has been proposed. This cam shaft is advantageous in wear resistance and heat resistance properties because of the chilled structure of a high hardness formed on the cam surface portion but insufficient in pitting resistance property.
A forged cam is also conventionally known. This type of cam shaft is provided after a heat treatment such as induction hardening, carburizing and the like. This cam shaft is advantageous in the pitting resistance property but disadvantageous in manufacturing cost. In addition, it is impossible to get a hollow cam structure through a forging process so that the forged cam is disadvantageous in weight.
Japanese Patent Publication No. 55-3422, published on Jan. 25, 1980, discloses a ductile cast iron consisting of 3.2-4.0% of C, 1.5-5.0% of Si, less than 0.08% of P, less than 0.02% of S, 0.02-0.08% of Mg, 0.10-0.26% of Mo, 0.3-1.4% of Mn with the balance of Fe in weight and the cast iron being formed by a bainite structure including 20-50% in volume of residual austenite structure. However, even if mechanical parts of the valve driving mechanism provided with the rocker arm with the roller are constituted by the cast iron disclosed the Japanese Patent Publication No. 55-3422, it is difficult to obtain a sufficient pitting resistance property.